20 amp wire size
A 20 amp circuit is normally wired with 12 AWG copper. That single size covers almost every 20 A branch circuit in a house, because 12 AWG copper has a 20 A rating in the 60 °C column used for NM-B cable and the NEC caps it at a 20 A breaker anyway.
Minimum wire size for 20 A
| Conductor | 60 °C column | 75 °C column |
|---|---|---|
| Copper | 12 AWG | 12 AWG |
| Aluminum | Not listed | 10 AWG |
Usually chosen: 12 AWG copper, as 12/2 NM-B with ground (yellow sheath) or as THHN in conduit. Aluminum branch-circuit wire this small is rarely installed in new work.
Breaker rating taken as the required ampacity; 240.4(D) limits applied. The charts list no 60 °C aluminum, and 60 °C copper only to 2 AWG (95 A), the largest NM-B. Manufacturer chart values (2017 NEC basis). Verify against the NEC edition adopted locally.
The reason a smaller wire will not do is NEC 240.4(D), which limits 14 AWG copper to 15 A overcurrent protection even though its 75 °C ampacity reads 20 A. The table below shows the minimum for each conductor material and column; the rest of the page covers where 20 A circuits are required, when long runs call for 10 AWG and the mistakes that show up most often at inspection.
Where 20 amp circuits are used
Several 20 A circuits are required in a dwelling rather than chosen. NEC 210.11(C)(1) calls for at least two 20 A small-appliance branch circuits serving the kitchen, pantry, breakfast room and dining room receptacles, so countertop appliances such as toasters, kettles and coffee makers can run together without tripping a 15 A breaker.
210.11(C)(2) adds at least one 20 A circuit for the laundry receptacles, and 210.11(C)(3) at least one 20 A circuit for bathroom receptacles, where hair dryers routinely draw 12 to 15 A. Recent editions also require a 20 A circuit for the receptacles in a garage that has electric power.
Beyond the required circuits, 20 A is the common choice for a dishwasher, a garbage disposal, a built-in microwave, a sump pump, a workshop bench and outdoor receptacles. A 240 V 20 A circuit, wired with 12/2 cable and a two-pole breaker, feeds baseboard heaters of up to about 3,800 W and small window air conditioners.
A 20 A circuit may carry 15 A receptacles when there is more than one receptacle on it, which is why kitchens are full of ordinary 15 A duplex devices on 12 AWG wire. A single receptacle on an individual 20 A circuit has to be rated 20 A.
Voltage drop at 20 A
At 120 V, 12 AWG copper loaded to the full 20 A stays within 3% only to about 45 ft one way. At 240 V the same wire reaches about 91 ft. Runs to a detached garage, a well pump or the far end of a long house are where the drop, not the ampacity, decides the size.
The table assumes the circuit carries its full 20 A, which most general-purpose circuits never do for long. For a circuit that will see a sustained heavy load, such as a space heater or a shop compressor, 10 AWG copper on the same 20 A breaker is the normal fix for a long run; upsizing the wire never requires a larger breaker.
| One-way length (ft) | Drop at 120 V (%) | Drop at 240 V (%) | Copper for 3% at 120 V | Copper for 3% at 240 V |
|---|---|---|---|---|
| 25 | 1.64 | 0.82 | 12 AWG | 12 AWG |
| 50 | 3.28 | 1.64 | 10 AWG | 12 AWG |
| 75 | 4.93 | 2.46 | 8 AWG | 12 AWG |
| 100 | 6.57 | 3.28 | 8 AWG | 10 AWG |
| 150 | 9.85 | 4.93 | 6 AWG | 8 AWG |
| 200 | 13.13 | 6.57 | 4 AWG | 8 AWG |
| 250 | 16.42 | 8.21 | 4 AWG | 6 AWG |
| 300 | 19.70 | 9.85 | 3 AWG | 6 AWG |
Full rating on a single-phase circuit, copper at 75 °C, stranded.
Breaker pairing
The breaker for 12 AWG copper is 20 A at most. Putting 12 AWG on a 15 A breaker is permitted and common where a circuit is upgraded later, but putting 14 AWG on a 20 A breaker is not.
A single piece of cord-and-plug equipment on a 20 A circuit may draw no more than 16 A (80%) under NEC 210.23, and fastened-in-place equipment on a circuit that also serves lights or receptacles is limited to 10 A. A continuous load, one that runs for 3 hours or more, is likewise limited to 16 A on a standard 20 A breaker.
NM-B cable or THHN in conduit
12/2 NM-B carries two insulated 12 AWG conductors and a bare ground; 12/3 adds a red conductor for a multiwire circuit or a three-way switch leg. Because NM-B is held to the 60 °C column, its 12 AWG conductors are rated 20 A, exactly the breaker size.
In conduit, 12 AWG THHN/THWN-2 is rated 25 A at 75 °C and 30 A at 90 °C, but the 240.4(D) limit still holds the breaker at 20 A. The extra ampacity is useful when more than three current-carrying conductors share a conduit: the 90 °C value is the starting point for the bundling adjustment, and 12 AWG often stays at 20 A even after derating.
Copper or aluminum
On the manufacturer charts, 12 AWG aluminum reads 20 A at 75 °C, but 240.4(D) limits 12 AWG aluminum to a 15 A device, so a 20 A circuit in aluminum would need 10 AWG, which 240.4(D) allows up to 25 A.
In practice aluminum is almost never used for 20 A branch circuits. Receptacles and switches would have to be marked CO/ALR, and the problems of older aluminum branch wiring from the 1960s and 1970s have made copper the default at this size.
Common mistakes
14 AWG on a 20 A breaker
The most common defect on 20 A circuits. A single run of 14 AWG anywhere in the circuit, often an extension added later, means the breaker must drop to 15 A. Cable sheath color helps spot it: white NM-B is 14 AWG, yellow is 12 AWG.
Using push-in terminals with 12 AWG
Many receptacles accept only 14 AWG in their push-in (backstab) holes. 12 AWG goes under the side screws or into a back-wired clamp terminal, wrapped clockwise and tightened to the marked torque.
Sharing the kitchen circuits
The two small-appliance circuits are for receptacles only. Range hoods, lighting and fixed appliances such as the dishwasher do not belong on them, apart from the few exceptions the NEC lists, such as a clock receptacle.
Ignoring the run length
A 20 A heater circuit run 100 ft in 12 AWG at 120 V loses more than 6% of its voltage. The heater still works but delivers noticeably less heat; 10 AWG keeps the drop near 4%.
Frequently asked questions
Can 14 gauge wire be used on a 20 amp circuit?
No. NEC 240.4(D) limits 14 AWG copper to 15 A overcurrent protection. Any 14 AWG conductor on a 20 A circuit requires the breaker to be changed to 15 A.
Is 10 gauge wire allowed on a 20 amp breaker?
Yes. A larger conductor than the minimum is always permitted, and 10 AWG on a 20 A breaker is a common way to limit voltage drop on a long run. The device terminals must accept 10 AWG.
How many outlets can go on a 20 amp circuit?
The NEC sets no fixed number of receptacles for dwelling branch circuits. Load calculations, the 210.23 limits on individual equipment and the circuits required for kitchens, laundries and bathrooms decide the layout; some local codes add their own limits.
Does a 20 amp circuit need 20 amp receptacles?
Not when the circuit has two or more receptacles: 15 A and 20 A receptacles are both permitted. A single receptacle on an individual 20 A branch circuit must be rated 20 A.
What size wire is used for a 20 amp 240 volt circuit?
Also 12 AWG copper. The conductor is sized for the current, not the voltage, so a 240 V 20 A circuit uses 12/2 cable with a two-pole 20 A breaker, with the white conductor re-marked as a hot conductor.